As described in the above-referenced '117 application, a variety of telecommunication test units have been proposed for testing digital signalling circuits. Such digital signaling circuits may include, but are not limited to, those transporting high bit-rate digital subscriber line (HDSL) signals, asymmetrical digital subscriber line (ADSL) signals, digital data service (DDS) signals and integrated services digital network (ISDN) signals. Unfortunately, such conventional test units are relatively cumbersome and hardware-intensive devices, that are customarily resident in a telcom service office or other fixed system installation, affording only a single point of access to the circuit to be tested.
A non-limiting example of such a conventional test unit is described in the U.S. Pat. No. 5,208,846, to Hammond et al, entitled: "Subscriber Loop Tester for Telephone Switching Systems." In order to be connectable with different types of telecommunication circuits, the Hammond et al test unit contains different types of line interfaces/jacks. Determining which jack is to be used requires that the test system operator have a priori knowledge of the communication link to which the test set is to be coupled. Without this knowledge, the user does not know to which jack the line should be connected, and cannot readily configure the test unit to support a particular user interface and associated termination hardware.
As noted above, because conventional test units are relatively large and fixed pieces of equipment, they are not readily suited for use in the field (i.e., they are not portable). As a consequence, they are not readily interfaced with any accessible location of a communication link. This relatively poor facility of transport and ready connection to any location along a link to be tested also limits the practicality and effectiveness of such test units.
These shortcomings have become a particular concern to telecommunication service providers, especially with the continuing expansion and demand for integrated services digital network (ISDN) services. Not only is there currently a need for a practical, portable test set that is capable of testing already installed ISDN circuits, but one which can perform `dry loop` line testing, prior to its use for ISDN service; namely, there is a need for an ISDN test device that will permit a circuit to be `prequalified` (as ISDN-capable), before ISDN equipment is purchased and connected to the line to be used for ISDN service.
Advantageously, the portable test set described in the above-referenced '117 application employs a communication architecture that is readily interfaced with virtually any location of a link/circuit to be tested, so as to allow testing of the link, irrespective of its configuration (e.g., two-wire vs. four-wire), or the type of signals that may be conveyed over the link (analog or digital). In addition to circuitry for testing the operation of a standard analog (POTS) line, the portable test set described in the '117 application contains a digital communications controller and associated digital signalling interface components, including both `U` interface and `S/T` interface circuits.
Thus, the test set can be interfaced with either a two-wire network link or a four-wire customer premises link, and may transmit and receive ISDN bearer channel messages over either type of circuit. The contents of the bearer channel messages may be defined to evoke prescribed responses from a companion test set coupled to another portion (relatively far end) of the communication circuit of interest, so that the circuit linking the two test sets may be tested. When two test sets are coupled to spaced apart locations of the circuit/line under test, they may operate in respective master and slave modes, allowing a craftsperson using the master test set to initiate a bit error rate test (BERT) from one end of the circuit, and derive a measure of the operational performance characteristic of the in-between segment of the circuit.